The present invention relates generally to a dry etching method.
In conventionally proposed dry etching methods, an object to be processed is mounted in a sealed processing chamber, and a processing gas is introduced into the processing chamber. Then, the processing chamber is evacuated to a reduced pressure atmosphere. Thereafter, high-frequency power is applied to an electrode formed in the processing chamber to excite plasma, and the object to be processed is etched by etchant ions in the plasma.
In such dry etching methods, etching gases used for etching a silicon oxide layer, e.g., a silicon oxide film (which will be hereinafter referred to as a "SiO.sub.2 film"), formed on a surface to be processed, of an object to be processed, e.g., a semiconductor wafer (which will be hereinafter referred to as a "wafer"), include, e.g., CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3. It is known that these gases are incombustible, low toxic and low reactive, so that these gases can be safely handled.
Recently CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3, c-C.sub.4 F.sub.8 have been noted in order to fabricate a semiconductor device of a hyperfine structure. For example, it is reported in "ULSI Dry Etching Technology Forum '96 opened on May 21, 1996" that c-C.sub.4 F.sub.8, which is a gas having a high C/F ratio (a ratio of C atomicity to F atomicity), is effectively used as a processing gas having both of anisotropy and selectivity in a high-density plasma.
If a processing gas introduced into a processing chamber has not contributed to a process, the processing gas is directly emitted to the atmosphere. The CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3 have infrared absorbing ability. It is also pointed out in "Climate Change 1995, IPCC, Cambridge Univ. Press" that these gases are very stable and are not easily decomposable if emitted to the atmosphere, and their lifetimes in the atmosphere are very long, 2,600 to 50,000 years, so that these gases cause the earth warming which maintains the green house effect for a long time.
As described in "Science, Vol. 259, Jan. 8, 1993", the reasons why the lifetimes of CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3 in the atmosphere are long are that these compounds rarely cause reactions, which are caused by usual compounds in the atmosphere, such as hydrolysis, oxidative decomposition and photolysis, and that radicals existing locally in the atmosphere, e.g., OH, H and O radicals, have very low reactivity with these compounds. It is also reported in "Nikkei Sangyo Shinbun dated on Jul. 1, 1996" that although it is required to take measures to solve the earth anathermal problem due to CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3, no measures have been taken.
As processing gases for SiO.sub.2 films in conventional sources of plasma of a low degree of dissociation, CHF.sub.3 is effective. However, in accordance with the recent miniaturization of semiconductor devices, the pressure of the plasma used for etching processes is decreased, and the density of the plasma is increased. Consequently, if CHF.sub.3, which produces a small number of F atoms by discharge dissociation using a source of plasma of a low degree of dissociation, is used for a high-density plasma source, a great number of F atoms are produced by a high gas decomposing efficiency. Moreover, it is pointed out in "ULSI Dry Etching Technology Forum '96 opened on May 21, 1996" that CHF.sub.3 is not suitable for etching processes of semiconductor devices, i.e., SiO.sub.2 films, which are required to be hyperfine-patterned, since the etch selectivity to Si and the etch selectivity to a resist are decreased if CHF.sub.3 is used for a high-density plasma source.
Therefore, in view of the prevention of the earth anathermal and the fine patterning of semiconductor devices, it is considered that as gases substituted for CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8, c-C.sub.4 F.sub.8 and CHF.sub.3, e.g., the same CF type of perfluoro olefins are used. However, most of perfluoro olefins are difficult to handle. In particular, C.sub.2 F.sub.4 (perfluoroethylene), c-C.sub.4 F.sub.6 (perfluorocyclobutene) and C.sub.4 F.sub.8 (perfluoroisobutene or perfluorobutylene) are very difficult to handle. For example, it is described in "12394 Chemical Products by Kagaku Kogyo Nippo-sha" that C.sub.2 F.sub.4 is an inflammable gas, and very sensitive to moisture, temperature and iron, and causes an explosive reaction. It is also described in "Fluorine Compounds, Kodansha Scientific" that c-C.sub.4 F.sub.6 has an acute toxicity equal to that of phosgene known as a toxic gas (LC50 is 20 ppm after exposure for 1 hour). Moreover, it is described in the same book that the acute toxicity of C.sub.4 F.sub.8 is ten times as large as that of phosgene.
However, it is described in "Material Safety Data Sheets, Occupational Health Services Inc." that among perfluoro olefins, most of which are difficult to be handled, only C.sub.3 F.sub.6 (perfluoropropene) is incombustible and low reactive. It is also described in "Fluorine Compounds, Kodansha Scientific" that C.sub.3 F.sub.6 (perfluoropropene) has a far lower acute toxicity than those of c-C.sub.4 F.sub.6 and C.sub.4 F.sub.8 since LC50 exposed for 4 hours is 3,000 ppm. Moreover, it is considered that the toxicity of C.sub.3 F.sub.6 is not so high since the high-pressure gas law, which provides that a toxic gas should be controlled in a permissible concentration of below 200 ppm, does not apply to C.sub.3 F.sub.6 as the toxic gas.
In addition, since it is described in "Material Safety Data Sheets, Occupational Health Service Inc." that C.sub.3 F.sub.6 is gradually hydrolyzed in the atmosphere, it is difficult to consider that C.sub.3 F.sub.6 is a compound having a lifetime of thousands years in the atmosphere. Moreover, since it is predicted from the descriptions of "Science, Vol. 259, Jan. 8, 1993" that substances gradually hydrolyzed in the atmosphere have a lifetime of years or less in the atmosphere, it is considered that C.sub.3 F.sub.6 is not a long-lifetime gas causing the green house effect.
As described above, C.sub.3 F.sub.6 scarcely causes the green house effect, and can be excellently handled. However, the technology for etching a SiO.sub.2 film using C.sub.3 F.sub.6 has scarcely been reported. In addition, all of currently reported technologies can not effectively carry out etching processes. For example, Japanese Patent Laid-Open No. 4-170026 discloses the technology for controlling the surface temperature of a substrate to be etched, so as to be below 50.degree. C. to etch the substrate using C.sub.3 F.sub.6. However, since the surface temperature of the substrate to be etched rises abruptly to about 100.degree. C. when the substrate is exposed to plasma during processing, it is required to provide a very large cooling facility in order to control the surface temperature of the substrate to be below 50.degree. C. Furthermore, since a resist is denatured if the surface temperature of the substrate to be etched is higher than 120.degree. C., it is required to control the surface temperature of the substrate to be below 120.degree. C.
In addition, Japanese Patent Laid-Open No. 61-133630 discloses an etching method using a mixed gas of C.sub.3 F.sub.6 and CHF.sub.3. However, it is pointed out in "Climate Change 1995, IPCC, Cambridge Univ. Press" that CHF.sub.3 is a gas causing the green house effect, and it is not considered that CHF.sub.3 is a practical processing gas. Moreover, comparative data on the etching characteristics of CHF.sub.3 and C.sub.3 F.sub.6 are described in "J. Vac. Sci. Technol., A12(3), May/June 1994" and "J. Vac. Sci. Technol., A12(4), July/August 1994". It is reported therein that, in the reactive ion etching for forming a contact hole and so forth, the etching rate of C.sub.3 F.sub.6 decreases by a greater value than that of CHF.sub.3 as the aspect ratio increases, so that the etching characteristic of C.sub.3 F.sub.6 is inferior to that of CHF.sub.3.